This invention relates to a method of purification by means of adsorption wherein the adsorbent is cyclically regenerated, in particular a method of the TSA (Temperature Swing Adsorption) type wherein a gaseous mixture comprising at least one main component and at least one impurity is at least partially purified.
Conventionally, a cycle of the TSA method comprises the following steps:                a) purifying the air via adsorption of the impurities at super-atmospheric pressure and at ambient temperature,        b) depressurising the adsorber to atmospheric pressure,        c) regenerating the adsorbent at atmospheric pressure, in particular via the waste gases, typically impure nitrogen coming from an air separation unit and heated to a temperature usually between 100 and 250° C. using one or several heat exchangers,        d) cooling to ambient temperature of the adsorbent, in particular by continuing to introduce said waste gas therein coming from the air separation unit, but not heated, and        e) repressurising the adsorber with purified air coming, for example, from another adsorber in production phase.        
It is known to regenerate the adsorbent with a gas that is different from the gas to be treated.
By way of example, U.S. Pat. No. 5,897,686 discloses the drying and the decarbonation of a H2/CO mixture by an inert, in particular nitrogen. The adsorber is then repressurised and put back into production.
However in certain cases, certain compounds even inert can be troublesome in the gas produced and must be excluded or remain under a given threshold so that the gas produced can be used industrially.
The final purification of so-called ultra-pure gases can be mentioned which must contain impurities only at the ppm, or even ppb level or even less.
This is also the case for certain applications of H2/CO mixtures or more generally of synthetic gas.
More particularly, the gaseous mixture is a synthetic gas including hydrogen, CO, possibly methane, nitrogen, argon, oxygen and the main impurities are part of the water group, methanol, CO2 with possibly NH3, HCN, traces of C2+ etc. hydrocarbons.
The purified gas is intended to be separated into fractions enriched with H2 and/or CO for example in a cryogenic unit or to be sent according to determined H2/CO ratios to downstream units.
As indicated, the synthetic gas can contain components in small quantities such as N2 or Ar that come either from natural gas or from coal used as a raw material, or from the oxygen used as an oxidant in the preparation of the synthetic gas, or from inlet air or from inert gas in the supply gas during its treatment.
These gases generally behave as inert gases, sometimes as poisons, according to the use which is made of the products resulting from the synthetic gas.
Their content can therefore be limited by specifications whether in the hydrogen, CO or H2/CO mixtures intended for diverse syntheses (oxo-alcohol, methanol, etc.).
These limitations can range from a few ppm to a few mol %, for example a few thousand ppm in hydrogen intended for refineries, 3% for certain syntheses.
A separation of these gases is possible but is generally expensive.
A solution, in the case of purifying synthetic gas, is to use a regeneration with hydrogen in order to prevent introducing other components, in particular nitrogen. However, this solution requires the regeneration gas to be recycled in order to avoid losing it.
As such, a problem that arises is to improve the purification methods having a regeneration with a gas containing an undesirable component, which would lead to introducing an excessive quantity of this undesirable component in the purified gas.